Frequency shift keying (FSK) is a digital modulation scheme. In the simple case of binary frequency shift keying e.g., the carrier wave frequency shifts between two frequencies for modulating the two symbol values of a binary digital baseband message. The term symbol denotes an element of a message. The symbol can take on only certain values, so called symbol values which are defined by the underlying digital system. A message is an assembly of symbols representing certain information.
A demodulation of a FSK modulated signal is done by a receiver which sequentially detects the received frequencies and translates these in the respective original symbol values. The demodulation can be implemented coherently or non-coherently.
A typical binary FSK receiver for coherent demodulation is shown in FIG. 1. The signal received at the antenna is bandpass filtered before being amplified by a low noise amplifier (LNA). The amplified signal may then be passed on to a down converter for translating the carrier frequencies to lower values. There are various forms of down converters and in some situations the down converter may also be omitted. The output of the down converter is then split and each component is mixed with a different oscillator frequency. For the binary FSK receiver of FIG. 1, the possibly down converted signal is split in two components with one being mixed with an oscillator frequency F1 and the other with an oscillator frequency F2. Each mixed component is supplied to a separate integrator which outputs are then compared for achieving a decision on the symbol value at the correct sampling time.
Non-coherent FSK receivers use baseband filters instead of e.g. phase coherent oscillators as shown in FIG. 2. In contrast to coherent detection, the decision on the symbol value is based on the detected baseband envelope instead of the integrator output for the coherent receiver.
When there are M frequencies used for a transmission of a digital message instead of only two, the modulation scheme is known a M-ary FSK, and there are M possibly transmitted signals. The number M of frequencies used for modulating a digital message defines the maximum number of possible symbol values allowed for a digital message to be modulated by an M-ary FSK system. But by modulating only a binary digital message, the transmission data rate increases due to the increased bandwidth resulting from the increased number of transmission frequencies.
Currently a variety of different FSK modulation techniques are known like e.g. Continuous Phase FSK (CFSK), Sunde's FSK and M-ary Orthogonal FSK as described in WO 03/0288255. CFSK is a form of FSK, in which there are no phase discontinuities in the transmitted signal. The transmitter can therefore be implemented as a single oscillator that is modulated by a data stream. Depending on the separation of the frequencies used to represent the individual symbol values, a respective system is referred to as norrowband or wideband system. A Sunde's FSK uses two frequencies for representing two symbol values, whereby the separation of the frequencies is the reciprocal of the data rate. In a M-ary Orthogonal FSK, M frequencies are used with the separation of the frequencies being the reciprocal of the data rate.
The known FSK communication systems use a fixed assignment of the transmission frequencies to the symbol values which may lead to signal interference particularly for a multipath propagation of the carrier wave and to a high bit error rate.
It is therefore an object of the present invention to provide a FSK modulation technique resulting in a reduced bit error rate.